Clive Page presented the following explanation :
These may be naive questions but I can't find answers anywhere. The context
is our energy supplier may well ask us to install a smart meter soon so I'd
like to understand how they work in practice.

I assume that they need some device both to replace or somehow connect to
both the existing gas and electricity meters (as well as another remote unit
to show the consumption). But where do these devices get their power from?
For the electricity meter that's no big problem: I assume the device,
whatever it is, will only use a watt or to to communicate to the home monitor
and to the power company, so that a simple tap of the mains will suffice,
after all that's how the existing meter works. But does the power that they
use come out of what the consumer pays for?

But what about the smart gas meter? The existing one works on gas pressure,
I assume. There's no electrical supply anywhere near our current gas meter -
which may well be a common situation. Are these smart devices also
gas-powered in some way or simply battery-powered? If so who is responsible
for noticing when the battery runs down, and then buying a replacement and
fitting it? I hope it's the power company, but if they have to make an
urgent visit every year or so to change a battery that is hardly going to
save them much compared to reading the existing meter at predictable
intervals. If it's the consumer's responsibility and you somehow don't
replace the battery does anyone notice or care? And does one's gas
consumption get lost during the period of battery outage? Suppose you go
away on holiday and the battery runs down just after you left, whose fault is
that?

These are all simple obvious questions, but I don't seem to have been able to
find answers. But I assume someone here already has a smart meter so will
know the answers to at least some of them.

Gas meter runs on a ten year battery and reports to the electric meter.
Electric meter then reports to utility company, both readings.